Conventional turbomolecular vacuum pumps include a housing having an inlet port, and interior chamber containing a plurality of axial pumping stages, and an exhaust port. The exhaust port is typically attached to a roughing vacuum pump. Each axial pumping stage includes a stator having inclined blades and a rotor having inclined blades. The rotor and stator blades are inclined in opposite directions. The rotor blades are rotated at high speed to provide pumping of gases between the inlet port and the exhaust port. A typical turbomolecular vacuum pump may include 9 to 12 axial pumping stages.
Variations of the conventional turbomolecular vacuum pump, often referred to as hybrid vacuum pumps, are known in the prior art. In one prior art configuration, one or more of the axial pumping stages are replaced with molecular drag stages which form a molecular drag compressor. This configuration is disclosed in U.S. Pat. No. 5,238,362, issued Aug. 24, 1993 to Casaro et al. A hybrid vacuum pump including an axial turbomolecular compressor and a molecular drag compressor in a common housing is sold by Varian, Inc. Other hybrid vacuum pumps are disclosed in U.S. Pat. No. 5,074,747 issued Dec. 24, 1991 to Ikegami et al.; U.S. Pat. No. 5,848,873 issued Dec. 15, 1998 to Schofield; and U.S. Pat. No. 6,135,709 issued Oct. 24, 2000 to Stones.
Molecular drag compressors include a rotor disk and a stator. The stator defines a tangential flow channel and an inlet and an outlet of the tangential flow channel. A stationary baffle, often called a stripper, is disposed in the tangential flow channel and separates the inlet and the outlet. As known in the art, the momentum of the rotor disk is transferred to gas molecules within the tangential flow channel, thereby directing the molecules toward the outlet. The rotor disk and the stator of the molecular drag compressor are separated by a small gap, typically on the order of 0.005 inch, selected to permit unrestricted rotation of the disk, while limiting leakage through the gap.
Prior art vacuum pumps which include an axial turbomolecular compressor and a molecular drag compressor provide generally satisfactory performance under a variety of conditions. Nonetheless, improvements are desired. One source of performance degradation that occurs in the molecular drag stages is backward leakage through the gaps between the rotor disk and the stator. In a specific example, gas may leak from the outlet of the molecular drag stage through the gap between the stationary baffle and the rotor disk to the inlet, thus reducing the achievable pressure ratio of the pumping stage. Leakage can be reduced by reducing the dimension of the gap between the stationary baffle and the rotor disk. However, a reduction in gap dimension requires increased precision and thereby increases cost. Furthermore, very small gaps increase the risk of undesired contact between the rotor disk and the stator during operation.
Accordingly, there is a need for improved molecular drag vacuum pumps which have a low level of backward leakage.